1. Field of the Invention
The present disclosure relates to a non-volatile memory cell comprising a reversible resistance switching metal oxide layer, its resistance value being representative of the data stored in the memory cell.
2. Description of the Related Art
Today the Flash non-volatile memory (NVM) technology dominates the market of non-volatile memories. In a flash NVM memory charge is stored in a conductive or non-conductive charge storage layer incorporated in a MOSFET structure. It is expected that this type of non-volatile memory technology will face severe scaling problems beyond the 45 nm technology node due to fundamental physical limitations associated with this data storage mechanism as put forward in the International Technology Roadmap for Semiconductors (ITRS), “2005 edition, Process integration, Devices and Structures” as downscaling the memory cell dimensions inherently leads to a reduction of the amount of charge, representative for a value of the bit, that can be stored in the memory cell.
Other non-volatile memory technologies are emerging which have the potential of allowing further downscaling of the memory cell dimensions. Among the most promising technologies is the resistive switching memory technology also known as Resistive Random Access Memory (ReRAM) technology. Such a ReRAM memory cell comprises a memory element and a selection element. The resistance of the non-volatile memory element can be reversibly varied between at least two stable resistance states employing a voltage- or current-induced change of the conductivity of the resistive switching material present in the memory cell. Each resistance state corresponds to a value of the bit stored in the memory cell.
Examples of such reversible resistive switching materials are chalcogenides, carbon polymers, selected binary metal oxides such as nickel oxide, tungsten oxide, copper oxide, ternary metal oxides such as nickel cobalt oxide or even more complex metal oxides such as Cr doped Sr(Ti)ZrO3 or Pr0.7Ca0.3Mn0.3.
Among the binary metal oxides nickel oxide plays a dominant role, as it forms a well characterized metal oxide having a stoichiometric nickel-to-oxide ratio 1/1. Nickel is a metal that is compatible with the mainstream CMOS technology. In a NiO based ReRAM memory cell, switching between two distinct resistance states is related to the formation and disruption of conductive filaments throughout the resistive switching NiO. As the filament width is believed to be in the range of nanometers, the NiO ReRAM memory cell can be scaled below the 45 nm technology node.
The unipolar resistive-switching of a NiO based ReRAM memory cell can be described as follows: 1) the cell resistance is initially high and requires a so-called electro-forming voltage to generate conductive filaments through the NiO matrix and to switch the cell to the low-resistive state (LRS), this electro-forming voltage is typically 3V or higher; 2) the created filaments can be disrupted in a sort of fuse blow using a high current, called reset current, so that the cell returns to a high resistive state (HRS), this reset current is typically in the milli-ampere range; 3) the filaments can be restored using a set voltage lower than the electro-forming voltage. A drastic reduction of these operating parameters is mandatory to allow scaling of the NiO-based ReRAM memory cell.
Kinoshita et al. discusses in “Universal understanding of direct current transport properties of ReRAM based on a parallel resistance model,” Journal of Material Research, Vol. 23, No. 3, March 2008 various models for the switching behaviour of binary metal oxides, in particular of a nickel oxide layer sandwiched between two platinum electrodes. In this paper the temperature and area dependency of the low resistance state (LRS) and the high resistance state (HRS) was studied.
A major drawback of the state-of-the-art NiO based ReRAM memory cells is that they require high electro-forming voltage and reset current.